1. Field of the Art
The present invention relates to an electric discharge type ozonizer.
2. Prior Art
Electric discharge type ozonizers having a basic structure as shown in FIGS. 3 to 7 have been put to practical use or proposed. In FIGS. 3 to 7, reference numeral 13 denotes a high-voltage electrode, 14 a grounding electrode, 15 a dielectric, 16 a discharge space, and 17 a high-voltage AC power supply.
FIG. 3 shows an ozonizer having a glass tube as a dielectric 15 and cylindrical electrodes 13 and 14. FIGS. 4 and 5 show ozonizers having a flat plate type dielectric 15 and electrodes 13 and 14. In the ozonizer shown in FIG. 4, a dielectric plate 15 is installed at one side of the electrodes only, whereas, in the ozonizer shown in FIG. 5, a dielectric plate 15 is installed at each side of the electrodes. FIG. 6 shows an ozonizer having serrated projections provided on the high-voltage electrode 13 in order to raise the electric discharge density. FIG. 7 shows a creeping discharge type ozonizer, wherein electric discharge is effected in a discharge space 16 formed between the both ends of the high-voltage electrode 13 and the upper surface of the dielectric 15. It is apparent that any of these ozonizers can be used even if the high-voltage side and the ground side are connected in reverse manner to the above.
Recently, ozone use has been rapidly expanding in the semiconductor manufacturing industry because of its noteworthy advantages; that is, its strong oxidizing action, decomposing action, catalytic action and its cleanliness. Ozonizers used in the semiconductor manufacturing process are required to produce high concentration of ozone and to minimize the content of contaminants in a gas containing ozone produced. Accordingly, oxygen gas, which is used as a raw material gas in electric discharge type ozonizers is also desired to be highly pure, having a purity not lower than 99.99%.
However, if oxygen of such high purity is used as a raw material gas in an electric discharge type ozonizer, the ozone concentration in the ozone containing gas produced decreases with time. Therefore, ozone gas having a high concentration cannot stably be obtained. The ozone concentration lowering phenomenon is peculiar to high-purity oxygen gas, and it does not occur with oxygen gas of about 99.5% purity. The decrease in ozone concentration with time is a phenomenon common to all electric discharge type ozonizers, which vary in structure, as described above, although there are some differences in the rate of decrease in ozone concentration.
In order to prevent a decrease in ozone concentration with time when ozone is generated by passing high-purity oxygen gas through an electric discharge type ozonizer, as described above, a method has been proposed in which an additive gas, e.g., nitrogen, argon, helium, carbon dioxide, etc., is mixed with high-purity oxygen gas used as a raw material gas.
More specifically, Japanese Patent Application Public Disclosure (KOKAI) No. 1-298003, for example, proposes a method wherein nitrogen gas of high purity is mixed with high-purity oxygen gas used as a raw material gas to produce ozone, which is used in a semiconductor device manufacturing process or other similar process, so as to prevent a decrease in ozone concentration with time in the formation of ozone by passing high-purity oxygen gas through an ozonizer, particularly an ozonizer having an electrode formed at the surface thereof with a dielectric comprising a glaze coat layer and a coating of alumina, quartz or the like, which is widely used in the process of manufacturing semiconductor devices. It should be noted that the specification of this publication mentions that when argon or helium of high purity is mixed with high-purity oxygen gas, it is impossible to effectively prevent a decrease in ozone concentration with time such as is attained when high-purity nitrogen gas is used.
Japanese Patent Application Public Disclosure (KOKAI) No. 3-218905 proposes a method wherein 0.02% to 2% nitrogen gas is mixed with high-purity oxygen gas which is to be passed through an ozonizer to produce ozone gas having a high ozone concentration used to form an insulating film on a semiconductor wafer.
Japanese Patent Application Public Disclosure (KOKAI) No. 1-282104 proposes a method wherein 1.0 vol % to 10.0 vol % an inert gas, e.g., nitrogen, argon, helium, carbon dioxide gas, etc., is mixed with high-purity oxygen gas which is to be passed through an ozonizer to obtain ozone gas used for the purpose of ashing organic contamination and a photoresist which are present on a silicon wafer as measures to prevent a decrease in ozone concentration with time.
However, if highly pure oxygen gas is mixed with another kind of gas for the purpose of preventing a decrease in an ozone concentration with time as in the above-described various cases, the additive gas mingles with the ozone gas produced, which produces an adverse effect on the semiconductor manufacturing process for which the ozone gas is used.
In addition, it is necessary to prepare a gas cylinder filled with a mixed gas as a raw material gas or to provide a means of mixing together highly pure oxygen gas and an additive gas, which make the ozonizer costly.
Particularly, when nitrogen is used as an additive gas, NO.sub.X gas (various kinds of nitrogen oxide gas), which is generated by electric discharge, is injected directly into the semiconductor manufacturing process, producing an adverse effect thereon.